The invention relates generally to audio mute systems and more particularly to audio mute control signal generating circuits.
More advanced digital audio functions are being planned for multimedia personal computers. For example, multimedia computers are proposed to include processing capabilities and mixing capabilities for multiple streams of audio received from a number of different audio sources such as digital versatile discs (DVD's), CDs, TV tuners, stereo analog inputs, auxiliary inputs, line inputs and other sources. Audio coders and decoders (codecs) are typically used to facilitate the digital signal processing of the audio signals from the various audio sources. High quality audio systems are in demand for such complex audio environments.
With the increase of additional audio streams being processed and mixed, a computer user may wish to mute or unmute an audio signal at any given time. For example, when a telephone call arrives through a modem and the computer is playing a movie on the screen of the computer from a DVD player, a user may wish to mute the audio from the DVD player so that a received telephone call may be heard. Therefore, muting or unmuting of the audio should be accomplished in a proper fashion so that no "clicks or pops" are heard over the computer speakers during a mute/unmute operation. When a mute state is desired, and the mute control system allows residual audio to pass prior to a complete mute, the residual audio can cause the "clicks or pops." To help reduce the residual audio, audio mute circuits typically attempt to detect the zero-crossing of an input audio signal so that the mute or unmute can occur upon the next zero-crossing to take advantage of a zero audio signal condition.
It is important in the computer systems that additional components such as audio control systems be small to help reduce cost and reduce the size of the overall system. Where the input audio is converted to differential input for zero-crossing detection, it is important that the differential converter be robust enough to accommodate parametric variations when the circuit is fabricated on an integrated circuit chip. However, problems can arise when the input audio to the differential converter is at a low level, such that the zero-crossing detection circuitry does not detect low audio levels. In such instances, the mute circuit will not properly mute or unmute the audio and will allow the low level audio to pass through to the speakers.
In traditional differential amplifier structures, process variations in low level input signals can tend to move the output common mode voltage of one differential amplifier stage below the threshold of an input transistor device of a cascaded second differential amplifier stage and hence typically cannot be used to drive an identical subsequent operational amplifier. The problem arises due primarily to integrated circuit process variations such that an output common mode voltage of the differential operational amplifier first stage can shift to a very low level causing an input stage of the second cascaded differential amplifier to be turned off completely when low input audio signals are present.
One traditional common mode feedback technique includes using a separate amplifier and associated resistors in a feedback loop as a comparator circuit to generate a bias voltage for a cascade transistor based on a reference voltage. However, such a technique can be too large and costly for small integrated circuits. Another type of conventional lower cost approach is to use a resistor network to generate an increase in output common mode voltage of the first stage. However, such techniques still require large resistors that take up large integrated circuit real estate and also provide less accurate control than the more complex comparator approach.
Consequently there exists a need for an audio mute control signal generating circuit that provides detection of low audio input signals and provides suitable zero-crossing detection and mute control to reduce the effects of residual audio.